1. Technical Field
The present invention relates to a nitride semiconductor substrate and a manufacturing method of the same, and particularly suitably relates to a gallium nitride substrate.
2. Description of Related Art
Group III-V nitride semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN), gallium aluminium nitride (GaAlN) have a sufficiently large forbidden band width, and an inter-band transition is a direct transition type. Therefore, examination is actively performed, regarding an application to a short-wavelength light emitting element. Also, application to an electronic element is also expected, because a saturation drift velocity of electrons is great, and 2-dimensional carrier gas by hetero-junction can be utilized.
A nitride semiconductor layer constituting these elements can be obtained by causing epitaxial growth on a base substrate, by using vapor-phase deposition methods such as a metal-organic vapor phase (MOVPE), a molecular beam epitaxy (MBE), and a hydride vapor phase epitaxy (HVPE). MOVPE and MBE are used in a case of obtaining mainly a thin film layer, and HVPE is used in a case of obtaining mainly a thick film layer. However, in the nitride semiconductor layer, there is no base substrate, with lattice constants matched with those of the nitride semiconductor layer. Therefore, it is difficult to obtain a high quality growth layer, and a plurality of crystal defects are contained in the obtained nitride semiconductor layer. The crystal defects are factors of inhibiting an improvement of element characteristics, and therefore examination has been actively performed heretofore, regarding reduction of the crystal defects in the nitride semiconductor layer.
As a method of obtaining a group III nitride crystal, with relatively less crystal defects, there is a method of forming a semiconductor multilayer film constituting an element part on a heterogeneous substrate, by using this heterogeneous substrate as a substrate for crystal growth. For example, a method of forming a low temperature deposition buffer layer on the heterogeneous substrate such as sapphire, and forming an epitaxial growth layer thereon, is known. In this crystal growth method using this low temperature deposition buffer layer, first, AlN or GaN is deposited on the substrate such as sapphire at around 500° C., and an amorphous film or a continuous film partially containing polycrystal is formed. The formed film is partially evaporated or crystallized by increasing a temperature to around 1,000° C., to thereby form a crystal nucleus with high density. If this crystal nucleus is grown as a nucleus of growth, a GaN film with relatively improved crystallinities can be obtained. In this case, the heterogeneous substrate such as sapphire remains as it is. However, even when the method for forming this low temperature deposition buffer layer is used, tremendous degree of crystal defects such as threading dislocations and vacancy exist in the obtained GaN film, and therefore it is insufficient in this method, to obtain an element having high performance which is desired at present.
In view of the above-described circumstance, a method for forming a semiconductor multilayer film constituting an element part on a GaN substrate by using this GaN substrate as a substrate for crystal growth, is actively examined. The GaN substrate for crystal growth is called a GaN freestanding substrate. However, as a method for obtaining the GaN freestanding substrate, ELO (Epitaxial Lateral Overgrowth: for example, see patent document 1) technique is known. The ELO method is a technique of obtaining the GaN layer with less dislocations by forming a mask having an opening part on the base substrate, and making the crystal laterally grown from this opening part. Patent document 1 proposes a technique of forming the GaN layer on a sapphire substrate by using this ELO method, then removing the sapphire substrate by etching, etc, to thereby obtain the GaN freestanding substrate.
As a method obtained by further developing the ELO method, FIERO (Facet-Initiated Epitaxial Lateral Overgrowth: for example, see non-patent document 2) method has been developed. The FIELO method shares a technique in common with the ELO method, in a point that selective growth is carried out by using a silicon oxide mask. However, this is a method for changing a propagating direction of the dislocations by forming a facet in a mask opening part during the selective growth, to thereby reduce the threading dislocations that reach an upper surface of the epitaxial growth layer. If the GaN thick layer is grown on the base substrate such as sapphire and thereafter the base substrate is removed, a high quality GaN freestanding substrate with less crystal defects can be obtained.
Also, as a method for obtaining the GaN freestanding substrate with low dislocations, DEEP (Dislocation Elimination by the Epi-growth with Inverted-Pyramidal Pits: for example see patent document 2) method is developed. By the DEEP method, GaN is grown, with silicon nitride patterned on not the sapphire substrate but the GaAs substrate as a mask, to thereby form a plurality of pits surrounded by a facet surface intentionally on a crystal surface, and the dislocations are accumulated in a bottom part of the pits. Whereby, the other area is low-dislocated.
In the aforementioned ELO method and DEEP method, crystal is grown, with the facet surface exposed to a crystal growth interface, at an initial time of the crystal growth. If the facet surface exists, an advancing direction of the propagated dislocations during crystal growth is likely to be curved. By utilizing such a behavior, the dislocations are set so as not to reach the crystal surface. Thus, the dislocation density on the substrate surface can be lowered. Further, when the crystal is grown on the crystal growth interface while producing the pits surrounded by the facet, the dislocations are accumulated with high dense on the bottom of the pit. When the dislocations are accumulated, mutually collapsed dislocations disappear, or an action such as stopping an advancement of the dislocations to a surface works by forming a dislocation loop. Thus, the dislocation density can be more effectively reduced.